Molecule-based microelectronic devices

ABSTRACT

Several types of new microelectronic devices including diodes, transistors, sensors, surface energy storage elements, and light-emitting devices are disclosed. The properties of these devices can be controlled by molecular-level changes in electroactive polymer components. These polymer components are formed from electrochemically polymerizable material whose physical properties change in response to chemical changes, and can be used to bring about an electrical connection between two or more closely spaced microelectrodes. Examples of such materials include polypyrrole, polyaniline, and polythiophene, which respond to changes in redox potential. Each electrode can be individually addressed and characterized electrochemically by controlling the amount and chemical composition of the functionalizing polymer. Sensitivity of the devices may be increased by decreasing separations between electrodes as well as altering the chemical environment of the electrode-confined polymer. These very small, specific, sensitive devices provide means for interfacing electrical and chemical systems while consuming very little power.

BACKGROUND OF THE INVENTION

The U.S. government has rights in this invention by virtue of ContractNo. N0014-75-C-0880 and Contract No. N00014-82-K-0737 from the Office ofNaval Research.

This is a continuation of U.S. Ser. No. 049,341, filed May 13, 1987, nowU.S. Pat. No. 4,895,705, which is a divisional of U.S. Ser. No. 674,410entitled "Molecule-Based Microelectronic Devices" filed Nov. 23, 1984 byMark S. Wrighton, Henry S. White, Jr., and Gregg P. Kittlesen, now U.S.Pat. No. 4,721,601, issued Jan. 26, 1988.

Presently available solid state microelectronic devices consist ofmicrocircuits with discrete circuit elements such as monolithicintegrated circuits, transistors, diodes, resistors, capacitors,transformers, and conductors mounted on an insulating substrate. Thinfilm hybrid microcircuits are formed by vapor deposition of conductors,such as copper and gold, and resistors, such as tantalum, nichrome, andtin oxide onto a passive or insulating substrate such as silicondioxide. An exact conductor pattern is obtained by masking orphotolithographic etching. The entire circuit is subsequently encasedwith an epoxy dip to protect against moisture and contamination.

Modern integrated circuit devices, even highly miniaturized very largescale integrated devices (VLSI), are responsive only to electricalsignals. There is now considerable interest in interfacingmicroelectronic devices with chemical and biological systems and it istherefore highly desirable to provide a microelectronic device that isresponsive to such chemical or biological inputs. Typical applicationsfor these devices include sensing of changes in pH and molarconcentrations of chemical compounds, oxygen, hydrogen, and enzymesubstrate concentrations.

Applicant is not aware of any apparatus or system which allows a directinterface between a microelectronic device sensitive to chemical imputsand a microminiature electrical circuit. Devices have been made on alarger scale which are sensitive to chemical input. These devicesinclude such well known apparatus as pH sensors. Work in this area hasrecently centered around the use of electroactive polymers, such aspolypyrrole or polythiophene. These compounds change conductivity inresponse to changes in redox potential. Recently, a polymericsemiconductor field effect transistor has been disclosed in a Japanesepatent, 58-114465. As described in this patent, polymers such astrans-polyacetylene, cis-polyacetylene, polypyrrole, and polyvinylphenylene have been used as inexpensive substitutes for single crystalsilicon or germanium in making a semiconductor field effect transistor.There is no recognition of the unique properties of these polymers inthis patent and, in fact, the polymers are treated as semiconductingmaterial even though the properties of the polymers are distinctlydifferent from that of silicon or germanium. The polymers are used assubstitutes for semiconducting materials sensitive to electrical signalsfor uses such as in memory storage. Disadvantages to the FET asdisclosed are that it is unstable and has a short useful life.

It is therefore an object of the present invention to provide a processfor producing microelectronic devices responsive to chemical input whichcan be incorporated into microelectronic systems which are responsive toelectrical input.

A further object of the present invention is to provide a process forconstructing molecule-based microelectronic devices on siliconsubstrates which can easily be integrated with solid state silicondevices for signal processing.

Still another object of the invention is to provide small, sensitive,and specific microelectronic devices with very low power requirements.

A further object of the invention is to provide diodes, transistors,sensors, surface energy storage elements, and light-emittingmicroelectrode devices which can be controlled by molecular-levelchanges in electroactive polymer components.

SUMMARY OF THE INVENTION

The present invention is a process for making microelectronic deviceswhich can be controlled by molecular-level changes in electroactivepolymer components. These devices are fabricated by functionalizingelectrodes formed by deposition of metal on silicon dioxide substratesusing conventional masking and phtolithography techniques with polymerswhose physical properties change in response to chemical signals. Thekey features are the small dimension of the electrodes and the smallspacing, in the range of less than five microns, between them.

In one embodiment, an analogue of a solid state transistor, wherein atransistor is defined as a material whose resistance can be adjusted byan electrical signal, is formed from an array of gold microelectrodesderivatized with a redox polymer such as polypyrrole. When polypyrroleis oxidized, it conducts an electrical current between themicroelectrodes. As in a solid state transistor, the current between thetwo outer microelectrodes of the array can be varied as a function ofthe potential of the polymer electrically connecting the electrodes in amanner analagous to the "gate" of a transistor. As the potential isaltered, the oxidation or reduction of the polypyrrole can be effected.This device amplifies the very small signal needed to turn thepolypyrrole from its reduced and insulating state to its oxidized andconducting state. Further variations are possible using additionalpolymers with different redox potentials.

In a second embodiment, a diode is fabricated on a silicondioxide-silicon substrate from an array of two or more microelectrodesseparated from each other by a distance of 2 microns or less,individually functionalized with a chemically responsive polymer, suchas a redox polymer. Examples of redox polymers are polypyrrole,poly-N-methylpyrrole, polythiphene, poly-3-methylthiophene,polyvinylferrocene, derivatized styrene and polyaniline. As manydifferent polymers may be used as there are pairs of microelectrodes.Since the polymers respond at different potentials, each pair ofelectrodes can be effectively isolated from the other microelectrodes.

In yet another embodiment, a microelectronic device with transistor or"triode-like" properties is fabricated by deposition of polyaniline ontoan array of two or more gold microelectrodes. Polyaniline, a redoxpolymer, has the unusual property of being insulating at an electricalpotential, less than +0.l V vs. SCE in aqueous 0.5M NaHS0₄, greater than10⁶ times more conducting at a slightly higher electrical potential,+0.4 V vs. SCE in 0.5M NaHSO₄, and insulating at a higher electricalpotential, +0.7 V vs. SCE in 0.5M NaHSO₄. The exact potential at whichthe polyaniline is conducting or insulating is determined by the medium,the amount of polyaniline connecting the electrodes, and interactionswith other polymers. This device is particularly useful as an electricalswitch between a specific range of potentials or as a pH or otherchemical sensor. The device may be further modified for use as an oxygenor hydrogen sensor by connecting the polyaniline to a noble metalelectrode such as a platinum electrode or by dispersing particles ofnoble metals such as palladium into the polyaniline.

Other specific embodiments of the present invention include surfaceenergy storage elements and light-emitting microelectrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a surface energy storage devicewherein electrical energy is used to charge the device by reducing apolyviologen polymer, (PQ^(2+/+))n, and oxidizing a polyvinylferrocenepolymer, (FeCp₂ ^(+/o))n.

FIG. 2 is a cross-sectional view of a molecule-based transistorconsisting of three gold microelectrodes, derivatized with polypyrroleand immersed in electrolyte, with a schematic showing how the electricalpotential of the gate is set using a potentiostat with a counterelectrode and a saturated calomel reference electrode (SCE).

FIG. 3 is a graph showing the output characteristics of the transistorof FIG. 2 as I_(D), the current between source and drain, as a functionof V_(D), the potential between source and drain, at various fixed gatepotentials, V_(G).

FIG. 4a is a cross-sectional view of a molecule-based transistor,consisting of two gold electrodes coated with polyvinylferrocene, (FeCp₂^(+/o))n, and polyviologen, (PQ^(2+/+))_(n), and functionalized with aquinone-based polymer, (Q/QH₂)_(n), having a pH-dependent redoxpotential which is more negative or positive than the potential of thviologen polymer, depending on the pH.

FIG. 4b is a schematic of the effect of pH variation on the polymers inthe transistor of FIG. 4a and shows the approximate relationship of theredox potentials.

FIG. 5 is a cross-sectional view of a molecule-based diode consisting oftwo gold microelectrodes derivatized with two polymers of differentredox potentials.

FIG. 6 is a cross-sectional view of an array of eight goldmicroelectrodes derivatized with different amounts of polypyrrole.

FIG. 7 is a graph of cyclic voltammograms at 100 mV/s for an array likethat in FIG. 6 in CH₃ CN/0.1M [n-Bu₄ N]ClO₄. The bottom portion of thesketch is the expected result based on the derivatization procedure andelectrochemical response.

FIG. 8a is a graph of the potential, V vs. SCE, measured in CH₃ CN/0.1M[n-Bu₄ N]ClO₄, of five gold microelectrodes connected with polypyrrolewhen one is under active potential control at -1.0 V vs. SCE and one isat a positive potential at which the polypyrrole is expected to beconducting.

FIG. 8b is a graph of the potential, V vs. SCE, of five goldmicroelectrodes connected with polypyrrole where only one electrode isunder active potential control.

FIG. 9 is a graph of the current, i, measured between electrodes, versusapplied potential, V_(appl) vs. SCE, for two adjacent microelectrodesconnected with polypyrrole as a function of V_(set), where V_(set) isthe fixed potential vs. SCE of one of the two electrodes, and V_(appl),where V_(appl) is the potential of the other electrode.

FIG. 10 is a graph comparing the diode characteristics for twomicroelectrodes connected with (a) polypyrrole and (b)poly-N-methylpyrrole where the fixed potential, V_(set), in (a) is +1.0V vs. SCE and in (b) is -0.6 V vs. SCE.

FIG. 11 is a cross-sectional view of a light-emitting pair ofmicroelectrodes wherein the two gold microelectrodes are connected by apolymer such that application of a voltage, approximately 2.6 V, resultsin emission of light characteristic of an excited tris, 2,2'-bipyridineruthenium (II) complex, Ru(bpy)₃ ²⁺.

FIG. 12 (inset) is a cross-sectional view of a device fabricated fromtwo polyaniline-coated gold microelectrodes wherein V_(D) is thepotential between one microelectrode "source" and another microelectrode"drain" at a fixed gate potential, V_(G), controlled relative to anaqueous saturated calomel reference electrode (SCE).

FIG. 12a is a graph of the drain current, I_(D), in microamps versus thedrain voltage, V_(D), in mV for the device shown in the inset at variousvalues of V_(G), where the charge passed in setting the gate to apotential where there is conductivity between source and drain can beregarded as an input signal.

FIG. 12b is a graph of I_(D) vs. V_(G) at a fixed V_(D) of 0.18 V forthe device shown in the inset.

FIG. 13 is a graph of a cyclic voltammogram at 100 mV/s for a devicesuch as the one described in FIG. 12 (inset) when V_(G) is +0.3 V vs.SCE and V_(D) is 20 mV. . . . is at 0 hours and . . . is after 16 hours.

FIG. 13 (inset) is a graph of I_(D) versus time in hours when V_(D) isat 20 mV, V_(G) is at +0.3 V vs. SCE, and the electrolyte is 0.5M NaHSO₄at pH 1.

FIG. 14a is a graph of the I_(D) vs. V_(G) for a device such as the oneshown in FIG. 12 (inset), where V_(G) is varied from -0.2 V vs. SCE to+0.8 V vs SCE.

FIG. 14b is a graph of resistance in ohms versus V_(G) for a device suchas the one in FIG. 12 (inset).

FIG. 15 is a graph for a device such as the one shown in FIG. 12 (inset)of I_(D) in microamps versus V_(D) in mV at a V_(G) of -0.2 V vs. SCE, apotential at which polyaniline is reduced and insulating.

FIG. 16 is a graph of I_(D) versus time in seconds at V_(D) of 0.18 Vfor a device such as the one shown in FIG. 12 (inset) for a V_(G) stepof -0.2 to +0.3 V vs. SCE.

FIG. 17 is a cross-sectional view of a polyaniline-connectedmicroelectrode array connected externally to a macroscopic indicatorelectrode.

FIG. 18 is a cross-sectional view of a polyaniline-connectedmicroelectrode array consisting of three gold microelectrodes connectedto a counter-electrode, reference electrode, and potentiostat.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a process for producing molecule-basedmicroelectronic devices consisting of two or more microelectrodesseparated by a small dimension, which can be contacted individually andindependently functionalized using electroactive polymers with specificproperties that are responsive to chemical and/or electrical signals.Examples of one group of electroactive polymers are redox polymers whichare insulating when reduced and conducting when oxidized.

The microelectrodes are small, typically on the order of 2 to 5 micronswide by 50 to 150 microns long by 0.1 to 0.15 microns thick, althougheven smaller electrodes may be utilized, and made of inert, electricallyconductive material such as gold, silver, palladium, gold-platinum, andgold-palladium or other metals that are electrochemically inert. Theconductor should be easily deposited and have low electrical resistance,good adhesion to the substrate, stability, and ability to befunctionalized.

These electrodes are positioned on an inert substrate. An example of apreferred substrate would be oxidized silicon wafers made by growing a4500 Angstroms to 10,000 Angstroms thick SiO₂ layer on <100> Si. Devicesmade according to the present invention on silicon wafers may be easilyintegrated into presently available solid state microelectronic devices,most of which are also produced on silicon wafers.

The small separation between electrodes, typically on the order of 0.1to 2 microns, combined with the use of electroactive polymers withspecific properties, is crucial to the invention. The smallestinter-electrode space technically feasible is preferred. The smallinter-electrode space allows high current densities. As the distancebetween microelectrodes is increased, output decreases and "noise"increases. The direction of current flow, the ability to respond to achemical signal such as a change in pH, the rate of response, the degreeof response, the storage of energy, and the ability to place other pairsof electrodes in close proximity without interference is due to thechoice, deposition, degree of separation and quantity of polymer.

Various groups of polymers known to those skilled in the art aresuitable for use in the present invention. The requirements for suchpolymers are that they can be electrochemically deposited on individualelectrodes and polymerized and that they can respond to a signal, in areversible manner, in a way which can be electrochemically detected.Such materials are described by R. W. Murray in ElectroanalyticalChemistry, Vol. 13, Edited by A. J. Bard (Marcel Dekker, N.Y., 1984).

Suitable electrochemically polymerizable materials for use in thepresent invention include redox polymers. Examples of such polymers arepolypyrrole, polyaniline, poly-N-methylpyrrole, polythiophene,poly-3-methylthiophene and polyvinylferrocene (poly vinyldicyclopentadienyliron). Styrene and vinyl aromatic derivatives such asvinyl pyridine, vinyl,2,2'-bipyridine and metal complexes of thesederivatives, are also useful since they can be electrochemicallypolymerized and may be derivatized with a number of reagents, includingbiologically active agents such as enzymes and ionophores that complexwith ions such as lithium and calcium.

Using two or more electrodes connected with one polymer, atransistor-like device may be fabricated. By choosing two or morepolymers with different redox potentials, adjacent electrodes may beelectronically isolated or made to function as diodes or surface energystorage units.

For polypyrrole and poly-N-methylpyrrole, the oxidized materials areelectronic conductors. The conductivity varies by more than 10¹⁰depending on the redox state of the polymers. The consequence of thevery large difference in conductivity with redox state is that thepotential drop can occur across a very small fraction of length of theconnecting polymer when one microelectrode is held at a potential wherethe polymer is reduced and insulating and the other is held at apotential where the polymer is oxidized and conducting. For example,polypyrrole is insulating at approximately -0.4 V vs. SCE potential butbecomes conducting at positive potentials up to any positive potentialat which the polypyrrole is durable. The actual conductivities of theoxidized polymers, measured in CH₃ CH/0.1M [n-Bu₄ N]ClO₄, of polypyrroleand poly-N-methylpyrrole, respectively, are approximately 10⁻² ohm⁻¹·cm⁻¹ and 10⁻⁴ to 10⁻⁵ ohm⁻¹ ·cm⁻¹.

In contrast to polypyrrole, polyaniline can be made conducting by eithera positive or a negative shift of the electrochemical potential, sincepolyaniline is essentially insulating at sufficiently netative (negativeof 0.0 V vs. SCE) or positive (positive of +0.7 V vs. SCE)electrochemical potentials. As a result, a polyaniline-based deviceresponds to a signal in a significantly different way from solid statetransistors where the current passing between source and drain, I_(D),at a given source to drain voltage, V_(D), does not decrease withincreasing gate voltage, V_(G). The conductivity of polyaniline has beenmeasured to span eight orders of magnitude and is sensitive to pH andother chemical parameters.

The potential at which a polymer exhibits a sharp change in conductivitydue to oxidation is the threshold potential, V_(T). V_(T) can bemanipulated by using different monomers or different redox polymers, andby varying the medium to be "seen" by the polymer.

Other polymers which are useful in the present invention include redoxpolymers known to be electrochromic materials, compounds which changecolor as a result of electrochemical reactions. Examples of suchmaterials are polyvinylferrocene, polynitrostyrene, and viologens.Viologens, described by Wrighton et al in U.S. Pat. Nos. 4,473,695 and4,439,302, the teachings of which are incorporated herein, are compoundsformed from 4,4'-bipyridinium which may be polymerized and covalentlybonded or otherwise confined to the surfaces of electrodes. Viologenssuch as dialkyl-4,4'-bipyridinium di-cation and associated anions,dichloride, dibromide, or di-iodide, form contrasting colors whenoxidized or reduced. Since each monomer unit of viologen has a 2+ chargewhich is balanced in the presence of two halide counter ions, thecounter ions can be replaced with a complex ion such as PtCl₆ ²⁻ whichcan then be reduced to yield embedded elemental Pt(O) in highlydispersed form. An enzyme such as hydrogenase can also be immobilizedonto or throughout the redox polymer to equilibrate the redox polymerwith the enzyme substrates.

Substituted viologens are useful for photogeneration of hydrogen fromaqueous electrolytes, for reduction of metal-containing macromolecules,and on p-type silicon photocathodes in electrolytic cells.

The invention is further illustrated by the following non-limitingexamples. Devices in these examples were constructed according to theprocedure outlined below, with minor variations.

FABRICATION OF MICROELECTRODE ARRAYS

Microelectrode arrays were fabricated in the Massachusetts Institute ofTechnology Microelectronics Laboratory in the Center for MaterialsScience and Engineering which includes a class 100 clean room and isequipped to meet the specialized requirements for the production ofsolid state microelectronic devices such as "silicon chips".

A two-mask process was designed. The first mask was made for a metallift-off procedure to form microelectrodes, leads, and contact pads. Thesecond mask was made to pattern a photoresist overlayer leaving a 50 to140 micron length of the microelectrodes and the contact pads exposed.

A microelectrode array was designed using the Computer Aided DesignProgram HPEDIT at a Hewlett Packard Model 2648A graphics terminal on aDEC-20. The design file was translated into Caltech Intermediate Form(CIF). This CIF file was translated to Mann compatible code and writtenon magnetic tape. Masks for photolithography were made from the file onmagnetic tape using a Gyrex Model 1005A Pattern Generator. E-K5"×5"×0.009" Extra Flat high resolution glass emulsion plates were usedto make the photolithography masks. The emulsion plates were developedby a dark field process.

p-Si wafers of <100> orientation, two inches in diameter and 0.011inches thick, obtained from Wacker Corp. were used as substrates uponwhich to fabricate the microelectrode arrays. The silicon wafers wereRCA cleaned in a laminar air flow hood in the class 100 clean room. Thewafers were immersed in hot aqueous 6% by volume H₂ O₂ /14% by volumeaqueous NH₃, briefly etched in hydrofluoric acid diluted 10:1 withdeionized water, immersed in hot aqueous 6% by volume H₂ O₂ /14% byvolume HCl, rinsed in deionized water (resistance greater than 14Mohm·cm), and spun dry. The cleaned wafers were loaded immediately intoan oxidation tube furnace at 1100° C. under N₂. For examples 1 to 5, adry/wet/dry/anneal oxidation cycle was used to grow a thermal oxidelayer 4500 Angstroms thick. For example 6, a dry oxidation cycle wasused to grow a thermal oxide 11850 Angstroms thick. Oxide thicknesseswere measured using a Gaertner Model L117 ellipsometer. The oxidizedwafers were taken immediately to the photolithography stage.

Each oxidized wafer was flood-coated with hexamethyldisilazane and spunat 6000 rpm for 20 sec. For examples 1 to 5, one ml of MacDermidUltramac PR-914 positive photoresist was syringed onto each wafer. Thewafer coated with resist was spun for 30 sec at 4000 rpm and thenprebaked 35 min at 90° C. For example 6, one ml of Shipley 1470 positivephotoresist was syringed onto each wafer and the wafer spun for 30seconds at 6000 rpm. The coated wafer was then prebaked 25 minutes at90° C.

A GCA Mann 4800 DSW Wafer Stepper was used to expose the photoresist.The Mann uses the 405 nm line of a 350 W Hg arc lamp as a light source.The mask image is reduced 5:1 in the projection printing. For examples 1to 5, an exposure time of 0.850 sec was used and the photoresistdeveloped 60 sec in MacDermid Ultramac MF-62 diluted 1:1 with deionizedwater. For example 6, the wafer was exposed for 1.2 seconds anddeveloped 60 seconds in Shipley 312 developer diluted 1:1 with dionizedwater. The developed wafers were then cleaned in a planar oxygen etchingchamber at 75-100 W forward power in 20 mtorr of oxygen for 15 seconds.

A bilayer metallization was performed. A MRC 8620 Sputtering System wasused in preparing the microelectrode arrays of examples 1 to 5. Thebilayer metallization of the wafers used in example 6 was performed in aNRC 3117 electron beam evaporation system. Wafers were placed on aquartz plate that was freshly coated with chromium. The wafers werebacksputtered 2 min at 50 W forward power in an argon plasma at 5 mtorr.Chromium was sputtered at 50 W forward power to produce a layer ofchromium. The layer on the wafers in examples 1 to 5 was 200 Angstromsthick. The layer in example 6 was 50 Angstroms thick. Gold was thensputtered at 50 W forward power to produce a layer 1000 Angstroms thick.Chromium serves as an adhesion layer for the gold. The combinedchromium/gold thickness of the wafers used in example 6 was measured tobe 1052 Angstroms on a Dektak II surface profile measuring device.

At this point, the chromium/gold was in direct contact with the SiO₂substrate only in the areas that were to form the microelectrodes,leads, and contact pads and on photoresist in all other areas. Thechromium/gold on photoresist was removed by a lift-off procedure: themetallized wafers were immersed in warm acetone, in which soft-bakedpositive photoresist is soluble, for 75 minutes for the wafers used inexamples 1 to 5 and 5 minutes for the wafers used in example 6. Thewafers used in examples 1 to 5 were briefly sonicated in acetone toremove the metal between microelectrodes, dried, and then cleaned ofresidual photoresist in a planar oxygen plasma etching chamber at 200 Wforward power in 50 mtorr oxygen for 60 sec.

The wafers used in example 6 was blasted with acetone from a Paasche airbrush with N₂ at 70 psi, sonicated for 30 minutes in acetone, thenrinsed with acetone and methanol before drying. The wafers were thencleaned in a mixture of hot aqueous 6% by volume H₂ O₂ /14% by volumeaqueous NH₃, rinsed in deionized water (greater than 14 megaohm.cm), andspun dry. The wafers were then baked at 180° C. for 40 minutes beforerepeating the photoresist spin coating process. The wafers were againprebaked at 90° C. for 25 minutes and then exposed in a Karl SussAmerica Inc. Model 505 aligner for 11 seconds, using a dark field mask.The photoresist was developed in Shipley 312 developer diluted 1:1 withdeionized water to expose the bond pads and the array of microelectrodewires. The exposed areas were cleaned of residual photoresist in theoxygen plasma etching chamber at 75-100 W for 1 minute. The remainingphotoresist was hardbaked at 180° C. for 15 hours.

Wafers were then baked at 180° C. for 40 minutes before repeating thephotoresist spin coating process. The wafers were again prebaked at 90°C. for 25 minutes and then exposed in a Karl Suss America Inc. Model 505aligner for 11 seconds, using a dark field mask. The photoresist wasdeveloped in Shipley 312 developer diluted 1:1 with deionized water toexpose the bond pads and the array of microelectrode wires. The exposedareas were cleaned of residual photoresist in the oxygen plasma etchingchamber at 75-100 W for 1 minute. The remaining photoresist was hardbaked at 180° C. for 15 hours.

Individual die (chips) were scribed and separated. The chips weremounted on TO-5 headers from Texas Instruments with Epoxi-Patch 0151Clear from Hysol Corp. A Mech-El Ind. Model NU-827 Au ball ultrasonicwire bonder was used to make wire bonds from the chip to the TO-5header. The leads, bonding pads, wire bonds, and header wereencapsulated with Epoxi-Patch 0151. The header was connected through aTO-5 socket to external wires. The external wires were encased in aglass tube. The header was sealed at the distal end of the glass tubewith heat shrink tubing and Epoxi-Patch 1C white epoxy from Hysol Corp.

Prior to use as a microelectrode array, the array was tested toestablish the leakage current between the various electrodes of thearray. Arrays characterized as usable have a measured resistance betweenany two electrodes of greater than 10⁹ ohms in non-aqueous electrolytesolution containing no added electroactive species. In many cases only afraction of the electrodes of an array were usable. Prior to use inexperimentation the microelectrode arrays were tested further in aqueouselectrolyte solution containing 0.01M K₃ [Fe(CN)₆ ] and 0.01M K₄[Fe(CN)₆ ] or with [Ru(NH₃)₆ ]Cl₃ to establish that the microelectrodesgive the expected response. Typically, a negative potential excursion toevolve H₂ cleaned the gold surface sufficiently to give goodelectrochemical response to the Fe(CN)₆ ^(3-/4-) or Ru(NH₃)₆ ^(3+/2+)redox couples. The electrolyte used for electrical measurement was 0.1MNaClO₄ in H₂ O solvent, 0.5M NaHSO₄, or 0.1M [n-Bu₄ N]ClO₄ in CH₃ CNsolvent.

ELECTROCHEMICAL EQUIPMENT

Most of the electrochemical experimentation in examples 1 to 5 wascarried out using a Pine Model RDE 3 bipotentiostat and potentialprogrammer. In cases where two microelectrodes were under activepotential control and a third was to be probed, a PAR Model 363potentiostat/galvanostat was used in conjunction with the Pine Model RDE3. All potentials were controlled relative to an aqueous saturatedcalomel reference electrode (SCE). Typically, electrochemicalmeasurements were carried out under N₂ or Ar at 25° C.

For example 6, most of the electrochemical experimentation was carriedout using a Pine Model RDE 4 bipotentiostat and potential programmer. Insome cases where only a single potentiostat was needed a PAR Model 173potentiostat/galvanostat and a PAR Model 175 universal programmer wasused. Potential step experiments were carried out using the RDE 4 with aTektronix type 564B storage oscilloscope as the recorder.

DERIVATIZATION OF MICROELECTRODES

In examples 1 to 5, the gold microelectrodes were functionalized byoxidation of 25-50 mM pyrrole or N-methylpyrrole in CH₃ CN/0.1M [n-Bu₄N]ClO₄. The polypyrrole was deposited at +0.8 V vs. SCE, and thepoly-N-methylpyrrole was deposited at +1.2 vs. SCE. The deposition ofthe polymer can be effected in a controlled manner by removing the arrayfrom the derivatization solution after passing a certain amount ofcharge. Electrodes were then examined by cyclic voltammetry in CH₃CN/0.1M [n-Bu₄ N]ClO₄ to assess the coverage of polymer and to determinewhether the polymer coated two or more electrodes resulting in a"connection" between them.

Prior to use as a microlectrode array, each microelectrode wire in thedevices used in example 6 was tested with an ohmmeter to make sure itwas not shorted to any other wire on the device. Then eachmicroelectrode was tested by running a cyclic voltammogram in 0.01MRu(NH₃)₆ ³⁺ /0.1M NaNO₃ /H₂ O. The microelectrodes were derivatized byoxidation of a stirred 0.44M aniline solution in 0.5M NaHSO₄ /H₂ O atpH 1. The polyaniline was deposited at +0.9 V vs. SCE. Electrodes werethen examined by cyclic voltammetry in 0.5M NaHSO₄ at pH 1 to assess thecoverage of polymer and to determine whether the polymer coated two ormore electrodes resulting in a connection between them. Macroscopic goldelectrodes were derivatized with polyaniline by the same procedure toaccurately relate the thickness of polyaniline to cyclic voltammetryresponse and the charge passed in the anodic deposition. Typically, aportion of the gold flag was covered with grease prior to depositing thepolyaniline over the exposed gold surface. The grease was then removedwith CH₂ Cl₂ to give a well defined step from gold to polyaniline.

EXAMPLE 1

In one embodiment of the present invention, depicted in FIG. 1, asurface energy storage device 10 is constructed from two goldmicroelectrodes 12, 3 microns wide by 140 microns long by 0.12 micronsthick, deposited on a 1 micron thick SiO₂ insulator 14 grown on a <100>Si substrate 16 and separated by a distance of 1.4 microns. Eachmicroelectrode is individually coated with electrochemically depositedand polymerized polymers, polyviologen 18 and polyvinylferrocene 20.Electrical energy can be used to charge the device by reducing thepolyviologen, the (PQ²⁺)_(n) polymer, and oxidizing thepolyvinylferrocene, the (FeCp₂ ⁰)_(n) polymer, according to thefollowing reaction: ##STR1##

EXAMPLE2

In another embodiment of the present invention, shown in FIG. 2, amolecule-based transistor 22 is fabricated from three goldmicroelectrodes separated by 1.4 microns, derivatized with polypyrrole24. Typical coverage of the polypyrrole is 10⁻⁷ mol/cm² of exposed gold,and the individual microelectrodes are electrically connected. Themicroelectrodes are wired so as to correspond to the drain 26, gate 28,and source 30 as in a conventional solid state transistor.

The properties of the device are characterized by immersing the devicein an electrolyte, CH₃ CN/0.1M [n-Bu₄ N]ClO₄, and measuring the current32 between source 30 and drain 26, I_(D), as a function of the potential34 between source and drain, V_(D), at various fixed gate potentials 36,V_(G). The results are shown in FIG. 3.

At values for V_(D) of less than 0.5 V, the device is "off" when V_(G)is held at a negative potential where the polypyrrole is expected to beinsulating and I_(D) is small. When V_(G) is moved to potentials morepositive than the oxidation potential of polypyrrole, approximately -0.2V vs. SCE, the device "turns on" and a significant steady-state valuefor I_(D) can be observed for modest values of V_(D). The close spacingof the microelectrodes allows an easily measurable current to passbetween the source 30 and the drain 26 when V_(D) is significant andV_(G) is above the threshold, V_(T). V_(T), the gate potential at whichthe device starts to turn on, is approximately equal to the redoxpotential of polypyrrole. For V_(G) more positive than V_(T), the valueof I_(D) increases at a given value of V_(D), in a manner consistentwith the increasing conductivity due to an increasing degree ofoxidation. At sufficiently positive values of V.sub. G, greater than orequal to +0.5 V vs. SCE, I_(D) becomes insensitive to further positivemovement of V_(G) at a given value of V_(D), a result consistent withmeasurements of the resistance of the oxidized polypyrrole coated on amicroelectrode array. A small range of V_(D) values (0 to 0.2 V) is usedto minimize electrochemical reactions at the source 30/polymer 24 anddrain 26/polymer 24 interfaces.

A fraction of 10⁻⁸ C of charge is required to obtain the maximumsteady-state value of I_(D) when V_(D) is equal to 0.2 V with thisdevice. The value of I_(D) achievable with the device is 4×10⁻⁵ C/s. Itis apparent from these results that a small signal to the gatemicroelectrode can be amplified in much the same way that a smallelectrical signal can be amplified with a solid state transistor. Themajor difference is that the turn on/turn off time in the molecule-basedsystem is dependent on the rate of a chemical reaction rather than onelectron transit times across the source to drain distance. For themolecule-based system, the properties such as V_(T) and minimum turn onsignal can be adjusted with rational variation in the monomer used toprepare the polymer. Use of smaller dimensions and materials other thanpolypyrrole can also lead to faster switching times.

EXAMPLE 3

As shown in FIG. 4a, a molecule-based pH sensor 40 can theoretically befabricated using a two microelectode array on a SiO₂ -Si substrate 42.

The two gold microelectrodes 44, 45 are coated with polyviologen 46,(PQ^(2+/+))_(n), and polyvinylferrocene 48, (FeCp₂ ^(+/o))_(n),respectively, and then overlaid with another polymer 50 with a differentpH dependent redox potential, such as a polyquinone, (Q/QH₂)_(n), whoseredox potential is above the redox potential of the polyviologen at highpH and between that of the polyviologen and polyvinylferrocene at lowpH.

The pH variation serves as the signal to be amplified. Varying the pHresults in a variation in current passing between the two goldelectrodes at a fixed potential difference with the negative lead to theviologen coated electrode. As shown by FIG. 4b, alteration of the pHchanges the redox potential of polymer 50. Low pH acts to make it easierto reduce polymer 50. Current can flow between source 44 and drain 45when the negative lead is attached to the polyviologen-coated goldmicroelectrode 44 and the positive lead is connected to thepolyvinylferrocene-coated gold microelectrode 45 and the redox potentialof the polyquinone is between the redox potentials of the two polymers46 and 50 coating source 44 and drain 45. At a fixed potentialdifference, the current passing between the two microelectrodes 44 and45 should depend on the pH of the solution contacting the polymer 50.

A pH sensor may also be fabricated by coating a microelectrode arraywith a polymer such as polyaniline. For a device consisting of two goldmicroelectrodes, 0.1 micron thick, 4.4 microns wide, and 50 micronslong, separated by a distance of 1.7 microns, coated with a layer ofpolyaniline approximately 5 microns thick, changes in the pH of thesurrounding medium markedly alter the conductivity. For example, thevalue of I_(D) at V_(D) equal to 20 mV and V_(G) of 0.2 V vs. SCE isreduced upon raising the pH of the solution, where I_(D) is the currentbetween one electrode and the next, V_(D) is the potential between thefirst and second electrode, and V_(G) is the potential between the twoelectrodes and a saturated calomel reference electrode. I_(D) at pH 1 isapproximately 10² times greater than at pH 6.

Polyaniline is limited to use with solutions of pH less than 6 topreclude irreversible chemical changes that occur at the higher pHvalues. However, other pH-sensitive redox polymers may be used tofabricate microelectrode pH-sensors for other pH ranges.

Numerous uses in chemical systems are possible for such sensing devices.For example, such a device may be used to detect subtle changes in pH ofaqueous solutions. Electrical signals generated by the device could bedirectly amplified and processed further.

EXAMPLE 4

A molecule-based diode 50, produced according to the present invention,is shown in FIG. 5. Microelectrodes 52 and 54 are each individuallycovered with polymers 56 and 58 having very different redox potentials.The current passes between the two heavily coated, connectedmicroelectrodes 52 and 54 as a function of the threshold potential ofthe diode, which is dependent on the redox potentials of the polymers.Electrons only flow from microelectrode 52 to microelectrode 54 due tothe large difference in the redox potentials of the two polymers 56 and58. For example, for a polyviologen/polyvinylferrocene diode, chargewill pass only when the negative lead of the applied potential isconnected to the gold electrode 52 coated with polyviologen 56 and thepositive lead is attached to the gold electrode 54 coated withpolyvinylferrocene 58. This reaction is shown as: ##STR2##

As shown in FIG. 6, it is possible to electrochemically depositelectroactive polymers 60 on individual electrodes 62a-h in variableamounts. The electrodes 62e-h which are bridged by the polymer 60 areelectrically connected: charge can pass from one microelectrode 62e toanother microelectrode 62f-h via conduction mechanisms of the polymer60. Connected electrodes are typically associated with coverages ofapproximately 10⁻⁷ mol polymer/cm² electrode. Addressing one electrodeoxidizes and reduces the polymer 60 over all of the electrodes 62e-h.

FIG. 7 shows the cyclic voltammetry of the polypyrrole modified array ofFIG. 6 in CH₃ CN/0.1M [n-Bu₄ ]ClO₄ containing no added redox activespecies. The unfunctionalized electrodes 62a, 62b, and the electrode62c, with a negligible amount of polypyrrole, lack the cyclicvoltammetry signal characteristic of an electrode-confined polymer.Immediately adjacent to the non-derivatized electrodes 62a-c areelectrodes 62d-h that show cyclic voltammograms characteristic ofelectrode-confined polypyrrole. The shape of the voltammogram is nearlythe same as for a macroscopic gold electrode derivatized in the samemanner. In addition, the potential of the oxidation and reduction peaksare as expected for the oxidation and reduction of polypyrrole.

Based on the integration of the charge passed upon cycling thederivatized microelectrodes 62 individually between the negative andpositive limits, it can be seen that controlled amounts of polypyrrole60 can be deposited on the electrodes 62. The same results, with theexpected differences in the oxidation and reduction potentials, wereshown using poly-N-methylpyrrole instead of polypyrrole.

FIG. 8a and 8b show the spatial potential distributions across apolypyrrole array 70 where one (FIG. 8b) or two (FIG. 8a) of theelectrodes is under active potential control. The entire array 70 wasimmersed in CH₃ CN/0.1M [n-Bu₄ ]ClO₄ and a biopotentiostat used toactively control the potential of one (FIG. 8b) or two (FIG. 8a)microelectrodes against a common reference and counter electrode in theelectrolyte solution.

The potential of one microelectrode 72 in the five electrode array 70was set at a negative potential of -1.0 V vs. SCE and the potential ofanother microelectrode 74 varied between 0.0 and 1.0 V vs. SCE.

As shown in FIG. 8a, the potentials of electrodes 76, 78, and 80 notunder active potential control are nearly equal to the positivepotential applied to electrode 74. Although a small potential drop ofapproximately 50 mV occurs over the 9 micron distance separatingelectrodes 74 and 80, the essential finding is that nearly all, up to1.8 V, of the potential drop occurs across a narrow region immediatelyadjacent to electrode 72 under active potential control at -1.0 V vs.SCE. The result is consistent with the difference in conductivitybetween the reduced and oxidized state of the polypyrrole, of which theconsequence is that the potential drop occurs across a very smallfraction of length of the connecting polymer when one microelectrode isheld at a potential where the polymer is reduced and insulating andanother is held at a potential where the polymer is oxidized andconducting. This would not be an expected result for a polymer with onlya moderate conductivity, such as those that exhibit redox conductivitywhere a linear change in concentration of redox centers across thethickness spanned by two electrodes at differing potentials would give apotential profile predicted by the Nernst equation.

FIG. 8b shows that when only one 82 of the microelectrodes is underactive potential control in the positive region, all of the electrodesare at the same potential as would be expected when there is anelectrical connection between them. When one of the microelectrodes isdriven to a negative potential, it would be expected that all wouldultimately follow. Upon reduction, however, the polymer becomesinsulating and the rate of potential following can be expected to beslower.

As shown by the current vs. potential data in FIG. 9, polypyrroleconnected-microelectrodes 90 behave in a diode-like fashion. Current vs.V_(applied) curves are shown as a function of the potential, V_(set), atwhich one 92 of the electrodes is fixed relative to the SCE. The currentmeasured is that passing between the two microelectrodes. The magnitudeof the current passing through one microelectrode is identical to thatpassing through the other microelectrode but opposite in sign.

When V_(set) is sufficiently positive, the current vs. V_(applied) curveis linear over a wide range of V_(applied). The resistance ofpolypyrrole from the slope of such plots is about 10³ ohms. Currentdensities exceeding 1 kA/cm² were observed. When V_(set) is sufficientlynegative, there is a broad range of the current vs. V_(applied) curvewhere there is insignificant current. Therefore, as shown in FIG. 10a, agood diode characteristic can be obtained using polypyrrole coated,closely spaced microelectrodes. The onset of current closely correspondsto the situation where the V_(appl). results in the conversion of thepolypyrrole from its reduced and insulating state to its oxidized andstrongly conducting state.

As shown in FIG. 10b, results using poly-N-methylpyrrole in place ofpolypyrrole in the array shown in FIG. 9 were similar except that thevalue of V_(set) necessary to obtain a current that is linear asV_(applied) is varied is more positive than with polypyrrole. Theresistance of the poly-N-methylpyrrole is 10⁵ to 10⁶ ohms. Both thehigher resistance and the more positive potential necessary to obtainthe conducting regime are consistent with the known differences betweenpolypyrrole and poly-N-methylpyrrole.

EXAMPLE 5

A light emitting device 98 may also be made according to the process ofthe present invention. As shown in FIG. 11, light is emitted from apolymer 100 overlaying two gold microelectrodes 102 on a silicondioxide-silicon substrate 104 when an electrical current is applied. Inthe depicted device, light characteristic of an excited Ru(bpy)₃ ²⁺species is emitted when a voltage of approximately 2.6 V is applied.

Polymers useful in a light emitting device according to the presentinvention can be polymerized from any monomers which areelectrochemiluminescent, such as vinyl derivatives of rubrene ordiphenyl anthracene.

EXAMPLE 6

A triode-like device was also constructed by electrochemical depositionand oxidation of a polyaniline film onto a microelectrode arrayconsisting of eight gold electrodes, 0.1 micron thick, 4.4 microns wide,and 50 microns long, each individually addressable and separated fromeach other by 1.7 microns.

The magnitude of the current passing between electrically connectedmicroelectrodes at a given applied potential depends on theelectrochemical potential of the polyaniline. In an electrolyte ofaqueous 0.5M NaHSO₄, the current at a fixed applied potential is maximumat an electrochemical potential of +0.4 V vs. SCE and declines by afactor of greater than 10⁶ upon reduction to a potential of +0.1 V vs.SCE or oxidation to +0.7 V vs. SCE.

The polyaniline-functionalized microelectrodes were examined by cyclicvoltammetry in 0.5M NaHSO₄ at pH 1 to assess coverage of the polymer andto determine whether the polymer coating two or more electrodes resultsin an electrical connection between them. Derivatization of theelectrode can be controlled by adjusting the amount of polyaniline byvarying the amount of charge passed in the electrochemicalpolymerization. At one extreme, the amount of polyaniline can be smallenough to derivatize the individual microelectrodes but not toelectrically connect them. At the other extreme, polyaniline can bedeposited in amounts sufficient to electrically connect all of themicroelectrodes.

Both a separate, unconnected microelectrode and multiple, connectedelectrodes show the same cyclic voltammogram at 50 mV/s in 0.5M NaHSO₄as does a single unconnected reference microelectrode at 50 mV/s in 0.5MNaHSO₄. This is consistent with one electrode being capable of oxidizingall of the polyaniline present on a single microelectrode or on multipleconnected microelectrodes. When adjacent derivatized microelectrodes arenot connected, the sum of the areas under the cyclic voltammograms forthe individual electrodes is the area found when the microelectrodes areexternally connected together and driven as a single electrode. Thethickness of polyaniline is not measured to be directly proportional tothe integrated cyclic voltammetry wave as it is for surface-confined,viologen derived polymers. This lack of direct proportionality may beattributable to morphological changes in the polymer with increasingthickness.

As shown in FIG. 12 (inset), a triode-like device 110 was constructed bycoating two adjacent gold microelectrodes 112, 114 with a five to 10micron thick electrochemically deposited and polymerized film ofpolyaniline 116. Measurements were made by immersing the device 110 inaqueous 0.5M NaHSO₄ at 25° C. under an inert atmosphere of N₂ or Ar.Devices constructed in this manner exhibit fairly long term stability.

As shown by the cyclic voltammogram in FIG. 13 for the device 110, theconnected pair of electrodes exhibits a nearly constant steady statecurrent between the two microelectrodes for at least 16 hours when V_(D)is 20 mV and V_(G) is 0.3 V vs. SCE. In general, devices can be used forcharacterization for several days without significant deterioration.

The conductivity of polyaniline which is immersed in an electrolyte suchas aquous 0.5M NaHSO₄ depends on the electrochemical potential, whichcan be varied by varying V_(G). As shown in FIGS. 14a and 14b, theresistance of polyaniline depends on its electrochemical potential. Theminium resistance is at an electrochemical potential in the vicinity of+0.4 V vs. SCE. Changes in resistance in excess of 10⁶ are routinelymeasured.

The minimum resistance for polyaniline is similiar to that forpolypyrrole connecting two microelectrodes spaced 1.4 microns apart, asshown in example 3. It is significantly different from polypyrrole,however, in that polyaniline is less conducting at potentials less thanor greater than +0.4 V vs. SCE. The change in resistance of polyanilineis essentially reversible for potentials less than +0.6 V vs. SCE.Potentials significantly more positive than +0.6 V vs. SCE yield anincrease in the resistance of the polyaniline when the potential isagain decreased to +0.4 V vs. SCE. The limit of positive appliedpotential is determined by O₂ evolution and limited durability of thepolyaniline. The limit of negative applied potential is determined bythe onset of H₂ evolution.

As shown in FIGS. 12a and 12b, the triode-like device 110 shows anincrease and then a decrease in I_(D) as V_(G) is varied from negativeto positive potentials, unlike conventional solid state devices whichshow an increase in I_(D) as V_(G) is varied until the I_(D) ultimatelylevels off at a constant, V_(G) -independent value. The charge passed insetting the gate to a potential where there is conductivity between thesource 114 and drain 112 can be regarded as an input signal. For thedevice 110, the charge necessary to completely turn on the device isapproximately 10⁻⁶ C.

Transconductance, g_(m), is determined by the equation: ##EQU1##

Using the data in FIG. 12a and 12b, the maximum value of g_(m) fordevice 110 is approximately 20 millisiemens per millimeter of gatewidth, as determined from the rising part of the I_(D) --V_(G) curve asV_(G) is moved to a potential more positive than approximately 0.1 mA/V.

By convention, gate length in Si/SiO₂ /metal field effect transistors(MOSFET) is the separation of source and drain. "Width" thereforecorresponds to the long dimension of the device 110. Since the g_(m) ofdevice 110 is only about one-order of magnitude less than that for goodMOSFET devices, the signal from the polyaniline-based device can be fedto conventional MOSFET in the form of voltage across a load resistancefor further amplification.

Diode-like behavior can be obtained using device 110, as shown in FIG.15, at V_(G) values where the polyaniline is reduced and insulating.Current passes between the microelectrodes 112 and 114 when the "source"microelectrode 114 is oxidized. If the "drain" microelectrode 112 ismoved to the negative of the source 114, current does not flow becausethe polyaniline remains insulating. Device 110 is not an exact analogueof a solid state diode because it is not a two-terminal device as is ap-n junction or a metal/semiconductor Schottky barrier. The diode-likebehavior of device 110 results from a chemical reaction of the polymer116 at a particular potential that causes a change in conductivity ofthe polymer 116.

Persistent diode-like behavior is obtained by maintaining onemicroelectrode, the drain 112, at a negative potential at which it isinsulating. Difficulties are encountered with degradation of thepolyaniline when the potential of the microelectrode is held at apotential positive enough for the polyaniline to be insulating, +0.7 Vvs. SCE, with the other microelectrode at a more negative potential.

Chemical-based devices depend on chemical reactions such as redoxreactions which occur relatively slowly compared to the turn on/turn offspeeds for solid state diodes and transistors. As shown in FIG. 16,device 110 can be turned on and off in less than one second. In FIG. 16,the value of I_(D) is shown for a potential step of V_(G) from -0.2 to+0.3 V vs. SCE then back to -0.2 V vs. SCE at V_(D) of 0.18 V. Bymonitoring the rise and fall of I_(D) of the potential steps, on to offtimes of less than 50 ms and slightly longer off to on times were shown.

The polyaniline-coated device 110 exemplifies the type of molecule-baseddevices that could be used as chemical sensors where the input signal tothe device is a redox agent that can equilibrate with the polyaniline116 to change the value of I_(D) at a given value of V_(D). Thespecificity of the device stems from the fact that only those redoxreagents that will bring the electrochemical potential of thepolyaniline to a value that will allow current to pass will be detected.Further specificity arises from the failure of the polyaniline to reactwith a particular given redox reagent. For example, polyaniline does notequilibrate with the H⁺ /H₂ redox couple. There is, however, rapidequilibration of polyaniline with one-electron outer-sphere redoxreagents such as Ru(NH₃)₆ ^(3+/2+), E^(o) ' approximately equal to -0.18V vs. SCE which is close to the E^(o) ' of H⁺ /H₂ at pH=1 ofapproximately -0.3 V vs. SCE.

Polyaniline also equilibrates with Fe(CN)₆ ^(3-/4-). For example,immersion of the polyaniline-based device 110 into a solution of aqueous0.5M NaHSO₄ containing the oxidant K₃ [Fe(CN)₆ ], E^(o') of [Fe(CN)₆]^(3-/4-) approximately equal to +0.2 V vs. SCE, turns the device "on".Immersion of the device into a solution of 0.5M NaHSO₄ containingRu(NH₃)₆ ²⁺ turns the device "off".

As depicted in FIG. 17, the change in resistance of the polyaniline witha change in electrochemical potential can be brought about by externallyconnecting the polyaniline-connected microelectrode array 120 to amacroscopic indicator electrode 122 that will respond to reagents 124other than outer-sphere reagents. When the indicator electrode 122 isplatinum, the microelectrode array 120 can be equilibrated with H⁺ /H₂since platinum equilibrates with H⁺ /H₂.

The device 130 in FIG. 18 is useful in characterizing the device of FIG.17 since the potentiostat 132 and counter-electrode 134 can be used toquantitatively establish the amount of charge that is necessary to turnon the device 130. This device differs from the device 110 shown in FIG.12A by the presence of an additional polymer-coated microelectrode andbecause the source and drain float.

It is also possible to chemically functionalize the polymer directly, asby the deposition of a metal such as palladium or a metal oxide onto thepolyaniline connecting the microelectrodes. Palladium provides amechanism for equilibrating the polymer with H₂ O/H₂ and O₂ /H₂ O.

The present invention may be embodied in other specific forms withoutdeparting from the spirit and scope thereof. These and othermodifications of the invention will occur to those skilled in the art.Such other embodiments and modifications are intended to fall within thescope of the appended claims.

What is claimed is:
 1. A microelectronic device comprising at least twoclosely spaced electrically conductive electrodes on an insulatingsubstrate overlaid with an electroactive polymer, wherein said polymeris insulating at a first redox potential conducting at a second morepositive redox potential, and insulating at a third, more positive redoxpotential, and an electrolyte, wherein the conductivity of theelectroactive polymer is altered through the electrolyte usingelectrochemical processes.
 2. The device of claim 1 wherein said polymeris polyaniline.
 3. The device of claim 1 wherein said polymer isresponsive to one electron outer-sphere redox reagents.